Device for indicating brake lining wear

ABSTRACT

Wear warning device for indicating wear in brake lining and drum in drum brakes for motor vehicles with automatic brake adjustment. In a drum brake with a lifter and adjustment devices (5), comprising an internally threaded lifter head (7) and a lifter screw (8) screwed therein, the wear warning device comprises a switch (20) supported by the lifter head, the switch having an axially displaceable pin (22) in contact with the thread of the screw (8). the screw is provided at its distal end with a depression (26) in the thread. When the pin moves down into the depression, the circuit in the switch is closed in order to activate a warning device.

The present invention relates to a device for indicating wear on thecooperating frictional surfaces in vehicle brakes, comprising anelectrical switch means disposed, upon a certain amount of wear on atleast one of the said frictional surfaces, to activate a warning device.

Devices for indicating brake lining wear, so-called wear-detectors, areused for example in drum brakes for trucks, which have devices forautomatically adjusting the play between the brake shoe lining and thewear surface of the drum in pace with the wear on the brake lining andthe drum.

The wear detector serves in this case to increase safety and help thevehicle owner to plan for replacement of brake linings, thus reducingboth down-time and shop costs.

A known type of wear detector comprises a sensor in the form of a switchcast in the lining material and connected to an electric circuit. Whenthe lining layer above the switch has been worn away exposing the switchand when it comes into contact with the metal in the drum, a circuit isclosed which activates a warning device, e.g. a warning light on theinstrument panel of the vehicle.

A wear detector of this type has a number of disadvantages. The sensorembedded in the lining must be of the disposable type and thus bedisconnected when the lining is replaced. It measures (indicates) onlylining wear, and this creates uncertainty as to the total wear (liningand drum wear) and how must adjustment can be done before replacement isnecessary. This is especially pronounced for asbestos-free linings whichare made of a material which causes more wear to the drums than asbestoslining. If, for example, the lining is only halfway worn but the drum isso worn that the combined total wear is such that the adjustment devicehas reached a position corresponding to the maximum stroke length of thebraking mechanism, there will still be no activation of the warningdevice in this previously known type of wear detector.

One purpose of the present invention is in general to provide a weardetector of the type described by way of introduction, which canindicate when the combined drum and lining wear amounts to apre-determined value, regardless of the individual wear to each of thecomponents, and which does not need to be disconnected or replacedduring normal lining replacement.

This is achieved according to the invention by virtue of the fact thatsaid switch means has a sensor means which is coordinated with a pair ofinteracting adjustment elements for adjusting play between the frictionsurfaces, said elements being adjustable to various positions relativeto each other, and in that the sensor means are disposed to sense therelative positions of the adjustment elements and, at a certainpre-determined setting, actuate the switch means to activate the warningdevice.

By coordinating, in accordance with the invention, the wear detectorwith the adjustment mechanism instead of with the brake lining, oneeliminates the uncertainty arising when only the lining wear ismeasured. The wear detector does not need to be disconnected when thelinings or drums are replaced and it can be placed in an environmentwhich is more protective than that of the lining material itself and, ifneeded, it can be easily replaced.

An additional purpose of the invention is to provide a wear detectorwhich is particularly suited to a vehicle brake where the adjustingelements consist of a screw and nut device, e.g. of the type used in aso-called "Z cam brake". This type of brake has an internally threadedlifter head, into which a lifter screw has been screwed. A furtherdevelopment of the invention described above for such a vehicle brake ischaracterized in that the sensor means is displaceably supported by thenut means and has a portion in contact with the threaded portion of thescrew, that its threaded portion is made with at least one interruptionin the uniformity of the thread, and that the sensing means, uponcontact with said interruption, is disposed to actuate the switch meansto activate the warning device.

Such a device also makes it possible to achieve in a simple manner amechanism blocking means, which after a certain amount of readjustmentafter activation of the warning device locks the screw and nutarrangement together. This prevents, for example, in a so-called "Z cambrake", the lifter head from being locked in an outer position in theautomatic adjustment. Such locking in an outer position would otherwisecause damage to the adjustment device, the brake shoes and the coversleeves of the brake. Furthermore, in this case the inner wear edge ofthe drum prevents removal of the drum, so that the drum must first becut into pieces for removal.

The invention will now be described in more detail with reference to anexample shown in the accompanying drawing, where

FIG. 1 shows a partial cut-away perspective view of a cam housing in a Zcam brake, which is known per se, with a wear detector according to theinvention, and

FIGS. 2 and 3 show the adjustment mechanism of FIG. 1 in a first and asecond end position, respectively.

In FIG. 1, I designates a cam housing in a drum brake of Z cam type, inwhich a shaft 2 is rotatably journalled. The shaft 2 can be rotated by abrake servo device (not shown) and is provided at its end with aso-called brake key 3 with pins 4 which extend into individual liftingand adjusting devices (generally designated 5).

As the shaft 2 is turned, the pins 4 push the devices 5 outwards,whereupon covers 6 abutting the ends of the brake shoes (not shown) pushthe brake shoes against the friction surface of the surrounding brakedrum (not shown).

Each lifting and adjusting device 5 comprises an internally threadedlifter head 7, to which the cover 6 is fixed. A lifter screw 8 isscrewed into the lifter head 7 and is fixed at one end to a sleeve 9,into which the pins 7 extend and which are externally provided withhelically cut grooves 10. A so-called adjuster spindle 11 with a gear 12with obliquely cut teeth in engagement with the grooves 10 in the sleeve9 is rotatably and axially displaceably mounted in the cam housing 1. Onthe adjuster spindel 11 a brake cone 13 is fixed. The brake cone 13 isnormally in contact with a seat (not shown) in the cam housing 1 andbrakes the spindel 11 against rotation. There is a small play betweenthe helical grooves 10 of the sleeve 9 and the teeth of the gear 12, andthis permits displacement of the lifter device 5 outwards to normallypress the brake shoes against the drum without there occurring anyrelative rotation between the sleeve and the gear.

When the linings and/or the drum have been worn so much that adjustmentis required, the lift devices 5 must be displaced further out in orderto press the brake shoes against the drum. The play between the teeth ofthe gear 12 and the helical grooves 10 in the sleeve is then taken upfirst and the adjuster spindle 11 is displaced axially, so that thebrake cone 13 is lifted from its seat, whereafter the spindle 11 isturned under the influence of the lifter device 5 when it continues tomove outwards. When the brake is released and the lifter device 5, underthe influence of springs (not shown) is moved back to its originalposition, the adjuster spindle 11, under the influence of the engagementbetween the teeth and the helical grooves, will first be displacedaxially, so that the brake cone 13 is pressed against its seat lockingthe spindle against rotation. As the lifter device 5 continues to move,the sleeve 9 under the influence of the blocked gear 12 will turntogether with the lifter screw 8, which is screwed relative to thelifter head 7, so that the effective length of the lifter device 5increases. Via a transverse shaft 14 with gears 15, the adjustmentmovement is transmitted from the lower to the upper lifter device 5.

FIGS. 2 and 3 show the minimum and maximum lengths, respectively, of thelifter device.

As is most clearly shown in FIGS. 2 and 3, the lifter head 7 is madewith a radial through-bore 16, in which an electrical switch means,generally designated 20, is fixed. The switch means 20 comprises amicroswitch 21, which is actuated by a pin 22, which is axiallydisplaceably housed in a sleeve 23 fixed in the bore 16. The pin 22 isspring-biased outwards, so that its end is kept in contact with thethread on the lifter screw 8. In the example shown, the pin 22 isdirected so that its end is in contact with the crest 24 of the thread,but as an alternative it can be directed towards the bottom between thethreads.

The thread crest 24 of the lifter screw 8 has been filed down at twoplaces so as to make a pair of interruptions 25, 26 forming depressionsin the thread crest 24. These interruptions or depressions 25, 26 areadapted to the pin 22 in such a way that the pin, when it rides on thethread crest 24, and is thus in its retracted position, breaks thecurrent in the microswitch 21 and, when it is in one of the depressions25, 26 and is thus in its extended position, closes the circuit in themicroswitch 21. Via a wire 27, the switch is connected to a warningdevice (not shown), for example an indicator lamp and/or a beep signal.

The outer depression 26 is placed relative to the pin 22 so that thepin, when it enters the depression, indicates, via the warning device,that the sum of the wear on the brake lining and the drum warrantslining and/or drum replacement. The depression 26 has a peripheralextension which permits a certain amount of further adjustment beforecomplete brake failure occurs, so that there is sufficient time to planservice. The end edge 28 of the depression 26, against which the pin 22is moved during automatic adjustment, can be made so that the pin, inits final contact with the edge 28, functions as a mechanical stop,preventing additional extension of the lifter device. This stop can thenreplace the stop ring, which is normally to be found at the end of thelifter screws of a Z cam brake. Such a stop ring 29 is shown in FIGS. 2and 3. By being able to eliminate this ring, this assembly of the lifterhead 7 for replacement or renovation is facilitated.

The inner groove 25 has nothing to do with the wear warning function.Rather, it is designed to indicate an inner end position, in which thecomponents are normally never at. An indication of the inner endposition could mean, for example, improperly adjusted brakes afterreplacing the brake lining.

I claim:
 1. A device for indicating wear on cooperating frictionalsurfaces in vehicle brakes, comprising an electrical switch means,disposed, upon a certain amount of wear on at least one of saidfrictional surfaces, to activate a warning device, said switch meanshaving a sensor means which is coordinated with a pair of interactingadjustment elements, characterized in that the adjusting elementscomprise a screw and nut arrangement which comprising a screw and a nutin threaded engagement with each other, that the sensor means (22) isdisplaceably supported by the nut (7) and has a portion in contact withthe threaded portion of the screw (8), that the threaded portion is madewith at least one interruption (26) in the uniformity of the thread, andthat the sensor means, upon contact with said interruption, is disposedto actuate the switch means (20) to activate the warning device. 2.Advice according to claim 1, characterized in that the sensor means (22)and the interruption (26) are so formed relative to each other that,upon contact with each other, they mechanically block the screw and thenut (7, 8) of the screw and nut arrangement (5) against relativerotation.
 3. A device according to claim 1, characterized in that thesensor means is formed of a pin (22) axially displaceable in a carrier(23) in the nut (7), said pin having an outer end in contact with thescrew thread and an inner end actuating the switch means (20) dependingon the axial displacement of the pin.
 4. A device according to claim 3,characterized in that the pin (22) is spring-biased towards and isarranged to ride on the thread crest, and that the interruption (26) isformed in a groove in the thread crest.
 5. A device according to claim3, characterized in that the pin is spring-biased towards and isarranged to slide in the thread bottom, and that the interruption isformed in a groove in the thread bottom.